A design of experiment study to investigate the effects of hardener concentration, stirring time, and air bubbles on the tensile strength of epoxy resin

Epoxy is one of the widely used polymeric resin systems in composite industries, which is a two-part resin system that requires the mixing of a hardening agent to trigger the polymerization process. The mixing ratio and pot-life are usually specified and provided by the manufacturer; however, the stirring time and the details of the mixing process were left to the fabricator to configure, based on some visual observations, which may vary from person to person. Although the mixing ratio is fixed by the resin manufacturer, errors may occur during the weighing and transfer of resin parts. The hardener concentration and stirring time are two of the most important factors that can affect the resin properties. In addition, air bubbles can be formed during the stirring process, which is inevitable. Design of experiment (DOE) is one of the widely used tools to design, control, and study the effects of multiple factors. In this research, DOE with a factorial design of 23−1 (2k−1) was used to study the effects of hardener concentration, stirring time, and air bubbles on the tensile strength of epoxy resin. Test specimens were fabricated, cut, and tested as per ASTM D638 standard (i.e., common test performed in the industry for plastics) in a randomized order and then the results were statistically analyzed using Design-Expert software. The test results showed that all three factors significantly affect the tensile strength of the epoxy, and they should be carefully optimized and used for the fabrication of composite materials with optimal properties.